Booting after the latest update to mysql, Kmail failed to proceed with the message:
“the akonadi personal information management service is not operational”.
Near the end of the error report (the saved one, not the error display) was the line:
stderr: "/usr/sbin/mysqld: Error on realpath() on ‘/var/lib/mysql-files’ (Error 2)
Creating the directory /var/lib/mysql-files has fixed the problem.
For background see https://forum.kde.org/viewtopic.php?f=215&t=135957
but note that it is a missing directory, not a file.